The subject of the invention is a process for the recovery of (1-S)-2-oxobornanesulphonic acid (D-camphor-.beta.-sulphonic acid) from contaminated aqueous solutions.
(1-S)-2-Oxo-bornane-10-sulphonic acid, hereinafter referred to for brevity as camphorsulphonic acid (further abbreviated CSA) is an important auxiliary acid in the resolution of racemates of amino compounds.
It has long been known that, for example, racemic aminoacids form diastereomeric salts with an optically active acid, and these salts can in many cases be resolved by fractional crystallisation (H. Bayer, Lehrbuch d. org. Chemic (Textbook of Organic Chemistry), 8th edition, 1961, page 258). The corresponding optically active aminoacids can be obtained from the resolved diastereomeric salts by decomposing them into their components. This principle is also the basis of the optical resolution of DL-2-phenylglycine with (1-S)-2-oxo-bornane-10-sulphonic acid, referred to in the past as D-camphor-.beta.-sulphonic acid, and hereinafter referred to for brevity as camphorsulphonic acid (further abbreviated CSA), which is described in Beilstein E III, 14, 1,187. (Timmermanns and Motiuk Bl. Soc. chem. Belg. 41 (1932), 402).
The preparation of an optical antipode by fractional crystallisation as a rule gives only moderate yields. In the process described for the preparation of D-2-phenylglycine, which is an important intermediate for the preparation of valuable penicillin and cephalosporin antibiotics, the yield achieved is also only from 30 to 35%, relative to the racemic aminoacids employed.
Using this process, equimolar amounts of DL-2-phenylglycine and CSA are dissolved in water at elevated temperatures and on cooling, the more sparingly soluble D-enantiomer crystallises out as the camphorsulphonate. From the mother liquor, the excess phenylglycine is recovered after alkaline racemisation and bringing the mixture to the isoelectric point, whilst the camphorsulphonic acid which is not co-precipitated and which is contaminated with large amounts of inorganic salts, is lost.
Even if, in a preferred procedure, 1/2 equivalent of the camphorsulphonic acid is replaced by hydrochloric acid, the loss of the valuable optically active acid remains considerable. In addition, the large amount of sodium chloride (potassium chloride) which, because of the nature of the process, passes into the waste liquors prevented the recovery of the CSA still present therein.
Such a process can only be carried out economically on an industrial scale if the valuable auxiliary materials of the process, which cannot be converted to the end product, are recovered.
The following procedures for the recovery of camphorsulphonic acid have already been described:
According to the process of U.S. Pat. No. 3,221,046
1. an aqueous CSA solution is treated, at pH 3 to 6, with a water-immiscible solvent which contains a secondary amine of the type of the "liquid amines mixture." In this treatment, the CSA is extracted into the organic phase and is reextracted therefrom into water at pH 9. By using a smaller volume of water than that of the starting solution, the CSA becomes concentrated. PA0 2. It is also known that acids, especially camphorsulphonic acid, can be isolated by the use of ion exchangers (G. N. Kulikova and J. T. Strukow, Pharm. Chem. J. 6, (1963) 6, 391-2). This process again presupposes that the CSA- containing solution is substantially free from inorganic salts. For the case of the splitting process described above, in which the resulting waste waters contain the camphorsulphonic acid in a very dilute form, alongside a four-fold to five-fold amount of inorganic salts, regeneration of the camphorsulphonic acid with ion exchangers is not realisable.
This process has the great disadvantage that it has only been described for CSA-containing solutions which are free from inorganic salts. The use of these "liquid amines mixture" with waste waters such as result from the particularly economically described diastereomer resolution process with subsequent alkaline racemisation, led to considerable difficulties as a result of stable emulsions which cannot be eliminated even by various modifications of the process.
Surprisingly, these disadvantages and limitations are overcome by the process according to the invention.